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Merge r185663 - SegmentedVector should waste less memory.
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<https://webkit.org/b/146069>

Reviewed by Anders Carlsson.

We were wasting sizeof(Vector) on every segment in SegmentVector.
The segments were using inline capacity, and would never go beyond it,
so all the size/capacity/out-of-line-buffer metadata was useless.

Change the internal representation to Vector<T[SegmentSize]> instead.
This saves 16 bytes per segment, so lower SegmentSize -> bigger savings!

* wtf/SegmentedVector.h:
(WTF::SegmentedVectorIterator::operator*):
(WTF::SegmentedVectorIterator::operator->):
(WTF::SegmentedVectorIterator::operator++):
(WTF::SegmentedVectorIterator::operator==):
(WTF::SegmentedVectorIterator::operator!=):
(WTF::SegmentedVectorIterator::SegmentedVectorIterator):
(WTF::SegmentedVector::at):
(WTF::SegmentedVector::append):
(WTF::SegmentedVector::removeLast):
(WTF::SegmentedVector::grow):
(WTF::SegmentedVector::begin):
(WTF::SegmentedVector::end):
(WTF::SegmentedVector::deleteAllSegments):
(WTF::SegmentedVector::ensureSegmentsFor):
(WTF::SegmentedVector::ensureSegment):
(WTF::SegmentedVector::allocateSegment):
(WTF::SegmentedVectorIterator::operator=): Deleted.
(WTF::SegmentedVector::SegmentedVector): Deleted.
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@carlosgcampos
Andreas Kling authored and carlosgcampos committed Jul 7, 2015
1 parent 733b409 commit 8e01153
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Showing 2 changed files with 68 additions and 46 deletions.
34 changes: 34 additions & 0 deletions Source/WTF/ChangeLog
View file
@@ -1,3 +1,37 @@
2015-06-17 Andreas Kling <akling@apple.com>

SegmentedVector should waste less memory.
<https://webkit.org/b/146069>

Reviewed by Anders Carlsson.

We were wasting sizeof(Vector) on every segment in SegmentVector.
The segments were using inline capacity, and would never go beyond it,
so all the size/capacity/out-of-line-buffer metadata was useless.

Change the internal representation to Vector<T[SegmentSize]> instead.
This saves 16 bytes per segment, so lower SegmentSize -> bigger savings!

* wtf/SegmentedVector.h:
(WTF::SegmentedVectorIterator::operator*):
(WTF::SegmentedVectorIterator::operator->):
(WTF::SegmentedVectorIterator::operator++):
(WTF::SegmentedVectorIterator::operator==):
(WTF::SegmentedVectorIterator::operator!=):
(WTF::SegmentedVectorIterator::SegmentedVectorIterator):
(WTF::SegmentedVector::at):
(WTF::SegmentedVector::append):
(WTF::SegmentedVector::removeLast):
(WTF::SegmentedVector::grow):
(WTF::SegmentedVector::begin):
(WTF::SegmentedVector::end):
(WTF::SegmentedVector::deleteAllSegments):
(WTF::SegmentedVector::ensureSegmentsFor):
(WTF::SegmentedVector::ensureSegment):
(WTF::SegmentedVector::allocateSegment):
(WTF::SegmentedVectorIterator::operator=): Deleted.
(WTF::SegmentedVector::SegmentedVector): Deleted.

2015-06-16 Andreas Kling <akling@apple.com>

Remove unused template parameter InlineCapacity from SegmentedVector.
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80 changes: 34 additions & 46 deletions Source/WTF/wtf/SegmentedVector.h
View file
Expand Up @@ -44,56 +44,41 @@ namespace WTF {

~SegmentedVectorIterator() { }

T& operator*() const { return m_vector.m_segments.at(m_segment)->at(m_index); }
T* operator->() const { return &m_vector.m_segments.at(m_segment)->at(m_index); }
T& operator*() const { return m_vector.at(m_index); }
T* operator->() const { return &m_vector.at(m_index); }

// Only prefix ++ operator supported
Iterator& operator++()
{
ASSERT(m_index != SegmentSize);
++m_index;
if (m_index >= m_vector.m_segments.at(m_segment)->size()) {
if (m_segment + 1 < m_vector.m_segments.size()) {
ASSERT(m_vector.m_segments.at(m_segment)->size() > 0);
++m_segment;
m_index = 0;
} else {
// Points to the "end" symbol
m_segment = 0;
m_index = SegmentSize;
}
}
m_index++;
return *this;
}

bool operator==(const Iterator& other) const
{
return m_index == other.m_index && m_segment == other.m_segment && &m_vector == &other.m_vector;
return m_index == other.m_index && &m_vector == &other.m_vector;
}

bool operator!=(const Iterator& other) const
{
return m_index != other.m_index || m_segment != other.m_segment || &m_vector != &other.m_vector;
return m_index != other.m_index || &m_vector != &other.m_vector;
}

SegmentedVectorIterator& operator=(const SegmentedVectorIterator<T, SegmentSize>& other)
{
m_vector = other.m_vector;
m_segment = other.m_segment;
m_index = other.m_index;
return *this;
}

private:
SegmentedVectorIterator(SegmentedVector<T, SegmentSize>& vector, size_t segment, size_t index)
SegmentedVectorIterator(SegmentedVector<T, SegmentSize>& vector, size_t index)
: m_vector(vector)
, m_segment(segment)
, m_index(index)
{
}

SegmentedVector<T, SegmentSize>& m_vector;
size_t m_segment;
size_t m_index;
};

Expand All @@ -110,10 +95,7 @@ namespace WTF {
public:
typedef SegmentedVectorIterator<T, SegmentSize> Iterator;

SegmentedVector()
: m_size(0)
{
}
SegmentedVector() = default;

~SegmentedVector()
{
Expand All @@ -125,7 +107,8 @@ namespace WTF {

T& at(size_t index)
{
return segmentFor(index)->at(subscriptFor(index));
ASSERT_WITH_SECURITY_IMPLICATION(index < m_size);
return segmentFor(index)->entries[subscriptFor(index)];
}

const T& at(size_t index) const
Expand All @@ -151,10 +134,9 @@ namespace WTF {
template <typename U> void append(U&& value)
{
++m_size;

if (!segmentExistsFor(m_size - 1))
m_segments.append(new Segment);
segmentFor(m_size - 1)->uncheckedAppend(std::forward<U>(value));
allocateSegment();
new (NotNull, &last()) T(std::forward<U>(value));
}

template<typename... Args>
Expand All @@ -166,15 +148,18 @@ namespace WTF {

void removeLast()
{
segmentFor(m_size - 1)->removeLast();
last().~T();
--m_size;
}

void grow(size_t size)
{
ASSERT(size > m_size);
ensureSegmentsFor(size);
size_t oldSize = m_size;
m_size = size;
for (size_t i = oldSize; i < m_size; ++i)
new (NotNull, &at(i)) T();
}

void clear()
Expand All @@ -186,12 +171,12 @@ namespace WTF {

Iterator begin()
{
return Iterator(*this, 0, m_size ? 0 : SegmentSize);
return Iterator(*this, 0);
}

Iterator end()
{
return Iterator(*this, 0, SegmentSize);
return Iterator(*this, m_size);
}

void shrinkToFit()
Expand All @@ -200,12 +185,16 @@ namespace WTF {
}

private:
typedef Vector<T, SegmentSize> Segment;
struct Segment {
T entries[0];
};

void deleteAllSegments()
{
for (size_t i = 0; i < m_segments.size(); i++)
delete m_segments[i];
for (size_t i = 0; i < m_size; ++i)
at(i).~T();
for (size_t i = 0; i < m_segments.size(); ++i)
fastFree(m_segments[i]);
}

bool segmentExistsFor(size_t index)
Expand All @@ -228,24 +217,23 @@ namespace WTF {
size_t segmentCount = (m_size + SegmentSize - 1) / SegmentSize;
size_t neededSegmentCount = (size + SegmentSize - 1) / SegmentSize;

// Fill up to N - 1 segments.
size_t end = neededSegmentCount - 1;
for (size_t i = segmentCount ? segmentCount - 1 : 0; i < end; ++i)
ensureSegment(i, SegmentSize);

// Grow segment N to accomodate the remainder.
ensureSegment(end, subscriptFor(size - 1) + 1);
for (size_t i = segmentCount ? segmentCount - 1 : 0; i < neededSegmentCount; ++i)
ensureSegment(i);
}

void ensureSegment(size_t segmentIndex, size_t size)
void ensureSegment(size_t segmentIndex)
{
ASSERT_WITH_SECURITY_IMPLICATION(segmentIndex <= m_segments.size());
if (segmentIndex == m_segments.size())
m_segments.append(new Segment);
m_segments[segmentIndex]->grow(size);
allocateSegment();
}

void allocateSegment()
{
m_segments.append(static_cast<Segment*>(fastMalloc(sizeof(T) * SegmentSize)));
}

size_t m_size;
size_t m_size { 0 };
Vector<Segment*> m_segments;
};

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